In Situ Growth of High Quality Epitaxial Yba 2 cu 3 O 7 - X Thin Films at Moderate Temperatures by Pulsed Laser Ablation
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IN SITU GROWTH OF HIGH QUALITY EPITAXIAL YBa2Cu307-x THIN FILMS AT MODERATE TEMPERATURES BY PULSED LASER ABLATION DOUGLAS H. LOWNDES, DAVID P. NORTON, J. W. McCAMY, R. FEENSTRA, J. D. BUDAI, D. K. CHRISTEN, AND D. B. POKER. Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. ABSTRACT Pulsed KrF (248 nm) laser ablation has been used for in situ growth of smooth, high-quality YBa2Cu 3 O7-x epitaxial films of variable thickness on SrTiO3, KTaO 3, LaGaO3, LaA103, cubic ZrO2, and MgO substrates, at temperatures of -600-730'C, without higher temperature postannealing. A rotating target pellet, line focusing by a single cylindrical lens, laser-beam scanning over the target, and laser energy densities -2.5-3 J/cm 2 can be combined to yield films of completely uniform composition and with -25% thickness variation over areas -8 cm 2 . The best films have Tc > 92 K and Jc(H = 0, T = 77 K) > 2 MA/cm 2 . Film-growth procedures are described, together with results of superconducting and normal-state transport measurements. INTRODUCTION High quality YBa2Cu3O7-x (Y-123) superconducting thin films can be grown by several methods, including electron beam co-evaporation [1,2], sputtering [3], and pulsed laser ablation (PLA) [4,5]. PLA has the advantages that smooth superconducting films having the same stoichiometry as the ablation target can be grown in situ at temperatures of only 600-750'C (i.e., without higher temperature post-annealing), and that this can be accomplished in a few minutes for films 100-200 nm thick (0.1-0.4 nm/pulse). A disadvantage of PLA is that the angular distribution of ablated material is strongly peaked (--coslO tO) in the forward direction [5] when the laser beam is focused to a small spot on the target, so that film thickness varies rapidly with position. However, both for accurate measurements of films' physical properties, and for possible technological applications of PLA ("scaleup"), it is desirable to produce smooth, single-phase, c-axis-perpendicular ("c-perp") films that have excellent compositional uniformity, and at least moderately good thickness uniformity, over larger areas. In this paper we describe the preparation and properties of high quality Y-123 films, using a laser beam that is brought to a line focus and scanned over the target perpendicular to the line-focus direction, in order to form an extended region of more uniform film deposition. Rutherford backscattering spectrometry (RBS) has been used to measure the compositional and thickness uniformity of deposits. Using this beam geometry, we have studied the effects on films' morphology, superconducting, and normal-state properties, of changes in the substrate temperature (Tsub), the position of substrates in the ablation "plume", and the post-deposition cooling rate and 02 pressure. PULSED LASER ABLATION FILM GROWTH AND CHARACTERIZATION A pulsed KrF (248 nm) excimer laser beam (1.1 J, -38 ns FWHM pulse duration) was passed through an aperture and brought to a horizontal line focus on a 2.5 cm-diam polycrystalline
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